Better way to combine it. Imagine you have enough time to learn js and .net together. In this time you can develop web applications with frontend and backend part and as background after 6 months you will know js framework (React, Angular, Vue, whatever) syntax and c# that you can use in mobile (react native + xamarin), software development (electron + .net core).
I don't think you can easily learn something so quickly, reason why you need time to understand how it works under the hood and know some shit moments.